Process and circuit for monitoring tire pressure

ABSTRACT

A process for monitoring tire pressure by comparing and evaluating the rotating speeds of the individual wheels of the vehicle which is based on the formation of speed correction factors. The values of the individual correction factors, correlations between the correction factors for typical travel situations, and tolerances for the correction factors, which depend on the actual travel situation, are determined during a learning phase (i.e. phase 1). The deviations of the correction factors from the learned values are determined and evaluated during a subsequent identification phase (i.e. phase 2), taking into account the travel situation-dependent tolerances and the progression of the learning process. Loss of tire pressure is signaled when the current speed correction factor determined for a wheel during the identification phase becomes smaller, taking into account the current deviation, than the speed correction factor already learned, taking into account the tolerance already determined for the current travel situation.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/EP94/03373.

BACKGROUND OF THE INVENTION

The present invention pertains to a process for monitoring the tirepressure of the wheels of a vehicle by measuring, comparing andevaluating the rotating speeds of the individual wheels of the vehicle.The present invention also includes a circuit for carrying out theprocess.

Processes for recognizing the loss of tire pressure, which are based onthe measurement and the comparison of the rotating speeds of theindividual wheels of the vehicle, already are known from U.S. Pat. Nos.4,876,528 and 5,192,929. In principle, the rotating speeds of thediagonally opposite wheels are added according to both of these patents,and the difference between the two sums is determined. If thisdifference is between 0.05% and 0.60% of the mean value of the two sums,the rotating speed of every individual wheel may deviate by at most 0.1%from the mean speed of all four wheels as long as a sufficient tirepressure is present. Loss of pressure is signaled in the case of greaterdeviations.

The accuracy of these prior art techniques leaves something to bedesired. When a spare tire or "emergency tire" with a different diameteris mounted, the method can fail completely.

Other processes, based, in principle, on the same algorithm, are knownfrom U.S. Pat. No. 3,581,277, JP 63-305011(A), and FR-25 685 19-A.

A process for evaluating the speed signals, which reflect the rotationbehavior of the individual wheels of a vehicle, is known from DE 39 15879-A1, in which a speed correction factor is determined for each wheel.The multiplication of the instantaneous wheel speed by the speedcorrection factor yields a common basic speed, which is derived from,e.g., the speed of the instantaneous slowest wheel. The wheel speedmultiplied by the corresponding correction factor is then used as thebasis for further signal processing, instead of the actual wheel speed.The process is used to determine the rotation behavior of a wheel withinthe framework of an antilock system.

SUMMARY OF THE INVENTION

An objective of the-present invention is to provide a process formonitoring the tire pressure, which is characterized by higher accuracyand reliability compared with the prior art processes, and which can beused even when tires, which have different rolling circumferences as aconsequence of manufacturing tolerances, uneven wear or the like, orbecause they are used only as emergency wheels, are mounted.

It has been found that this objective can be accomplished by a processin which velocity correction factors are formed for the individualwheels of the vehicle, and these factors, multiplied by the wheel speed,yield a basic speed, and the values of the individual correction factorsare determined in a learning phase. Correlations are determined betweenthe correction factors for typical driving situations. Band widths or"tolerances" that depend on the actual travel situation are determinedfor the correction factors. The deviations of the correction factorsfrom the learned values are determined and evaluated in anidentification phase, taking into account the tolerances that depend onthe travel situation and the progression of the learning process. Lossof tire pressure is signaled when, taking the current deviation intoaccount, the current correction factor for one wheel becomes smallerthan the speed correction factor already learned during the learningphase for the corresponding wheel for the current travel situation,taking into account the tolerance already determined for the currenttravel situation during the learning phase.

The process according to the present invention for monitoring the tirepressure is consequently based on the formation and the evaluation ofspeed correction factors, which more or less compensate for thedifferences in the diameters of the individual wheels, and on thedivision of the process into a learning phase and an identificationphase. The values of the individual correction factors and correlationsbetween these factors in typical travel situations, e.g., duringstraight travel or travel in curves, are first learned. The learningphase is followed by the identification phase, during which thedeviations from the learned correction factors are evaluated, takinginto account the travel situation. As soon as the learning process hasprogressed to the extent that the value of the correction factor hasbeen determined with high accuracy, and a travel situation, e.g., normalstraight travel, in which only slight deviations of the correctionfactors are possible in the case of intact tires, prevails, loss of tirepressure or the resulting reduction in diameter can be recognized withhigh accuracy.

According to an advantageous embodiment of the present invention, lossof tire pressure is signaled when the following condition is satisfiedfor a wheel:

    K.sub.x (t,FS)+G2.sub.x (t,FS)<K.sub.Sp,x (FS)-G1.sub.Sp,x (FS),

in which

x is wheel 1, wheel 2, wheel 3, wheel 4,

FS is the travel situation,

K_(x) (t,FS) is the current correction factor determined for wheel xduring the identification phase (phase 2),

K_(Sp),x (FS) is the speed correction factor for wheel x expected forthe current travel situation FS, which was already determined in phase1,

G1_(Sp),x (FS) is the tolerance already determined in phase 1 for thecurrent travel situation FS, and

G2_(s) (t,FS) is the current deviation for wheel x in phase 2.

The typical travel situations, in which correlations between thecorrection factors are determined and evaluated, include, e.g., one ormore of the following situations: travel in curves, straight travel,high acceleration or deceleration of the vehicle, rough road section,high drive slip or wheel slip while braking, high or low coefficient offriction, difference in the coefficients of friction on the right andleft sides, etc.

Relevant correlations are formed, e.g., by a comparison of thecorrection factors in pairs, axle by axle, diagonally or side by side,i.e., by all or some of these measures.

The correction factor tolerances are preferably evaluated according toaccuracy values, which are determined during the learning phase. Thesensitivity thresholds, which lead to the signaling of a pressure losswhen the deviations exceed these thresholds, are varied, according toone exemplary embodiment of the present invention, as a function of thetravel situation or as a function of the travel situation and theprogression of learning.

Provisions are also made according to the present invention to start thelearning phase by predetermined events, such as the mounting of a wheel,reset signal, and/or by regularly recurring events, such as the start ofthe vehicle or engine, expiration of a predetermined period of time,etc.

According to one exemplary embodiment, the learning phase is concludedwhen the correction factors reach a predetermined accuracy threshold,either taking into account the actual travel situation or under theassumption of ideal conditions.

The identification phase is started only after the conclusion of thelearning phase, or as soon as a minimum accuracy threshold has beenreached during the learning phase.

According to another exemplary embodiment of the present invention, itis possible to preset a plurality of accuracy thresholds, and thesensitivity threshold is varied as a function of the accuracy thresholdreached during the learning phase.

A circuit for carrying out the process according to the presentinvention is provided with a low-pass filter circuit, which is used forforming the speed correction factors on the basis of the wheel speedsignals, and whose attenuation characteristic is variable as a functionof the travel situation, which is reflected by the output signal of atravel situation recognition signal. In addition, there are circuits fordetermining the tolerances that depend on the travel situation andcircuits for determining the deviations from the learned speedcorrection values.

Further features, advantages and possible applications of the presentinvention will appear from the following explanations of details of thepresent invention taken in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of the most important components of acircuit for carrying out the process according to the present invention,

FIG. 2 is a simplified schematic representation of the most importantsteps of a first phase (learning phase) of the process according to thepresent invention, and

FIG. 3 is a simplified schematic representation of the most importantsteps of a second phase (identification phase) of the process accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the four wheels of a motor vehicle are designated by thesymbols VR, VL, HR, HL. Each wheel is equipped with a wheel sensor S1,S2, S3, and S4 which sends a signal, whose frequency and/or amplitudereflects the speed of the associated wheel.

An electronic circuit 1 is used to process and evaluate the sensorsignals and to generate a warning signal. Besides the inputs E1 throughE4 for the signals of the sensors S1 through S4, this electronic controlcircuit has additional inputs E5, E6, En for receiving additionalinformation needed for the tire pressure monitoring process or forstarting the process. In the exemplary embodiment shown, a steeringangle sensor LW, which sends a signal that is important fordistinguishing travel in a curve from straight travel, is connected toan input E5. An input E6 for a brake light switch BLS is indicated aswell. Additional inputs En are used to connect additional sensors,external memories for values already learned, etc., depending on theembodiment and the mode of application of the present invention.

Wheel sensors of the above-described type and electronic circuits forevaluating and processing the signals, which can also be used, at leastpartially, for monitoring the tire pressure, are already present invehicles equipped with antilock or drive slip control systems. Therequired additional expense for tire pressure monitoring according tothe present invention becomes minimal if such control systems arepresent.

FIG. 2 is used to illustrate phase 1, i.e., the learning phase, of theprocess according to the present invention. The wheel speed signalsv(t), obtained by means of the wheel sensors S1 through S4 (see FIG. 1)and processed, are sent for this purpose to a low-pass filter circuit 2.Speed correction factors K(t), which are an indicator of the deviationof the wheel speeds of the individual wheels from a basic speed, aredetermined by means of such filter circuits in a known manner, as isdescribed in DE 39 15 879 A1 cited above. By definition, the speedcorrection factors are values which, multiplied by the correspondingwheel speed, yield the basic speed. A basic speed common to all wheelscan be formed. The correction factors are now determined in a learningprocess, in which the change compared with the preceding cycle isdetermined during each calculation cycle, and evaluated for correctingthe current speed correction factor.

As is shown in FIG. 2, the speed correction factors K(t,FS) are obtainedby means of low-pass filter circuit 2, whose attenuation characteristic("ATTEN.") is a function of the driving situation. The vectorialnotation ##EQU1## expresses the fact that an individual speed correctionfactor applies to each wheel.

Whether travel in a curve, straight travel, low coefficient of frictionor high coefficient of friction, wheel slip or drive slip or anothertypical travel situation is present is recognized in a travel situationrecognition circuit 3 for recognizing the travel situation from thespeed signals v(t) of the individual wheels, using additionalinformation fed in via lines 4 and 5 shown in broken line in thisexample. The travel situation recognition signal is sent to the low-passfilter circuit 2, on the one hand, and, on the other hand, to atolerances circuit 6, which is used to determine or estimate travelsituation-dependent band widths or "tolerances" G1(t,FS) for the speedcorrection factors.

The correction factors K(t,FS) of the individual wheels are formed inthe low-pass filter circuit 2 in a learning process. The learningprocess depends on the travel situation FS. For example, the correctionfactors are learned more rapidly during straight travel with "normal"acceleration, i.e., outside the phases of high wheel slip or startingslip, than during travel in a curve, on a rough road section, etc. Verygenerally, the following formula applies:

    K.sub.1 (n+1)=K.sub.1 (n)+L(V.sub.Basis,1 -K.sub.1 (n)×V.sub.1),

in which "K₁ " is the correction factor for wheel "1", "n" and "n+1"indicate two consecutive scanning processes, L is the travelsituation-dependent parameter, which equals, e.g., 10⁻⁵ in the case ofstraight travel and becomes much smaller, e.g., 10⁻⁷ or 10⁻⁸ in other,unfavorable situations (curves, rough road section, etc.).

Correlations are determined according to the present invention betweenthe speed signals v(t) or correction factors K(t) of the individualwheels for the typical travel situations. To do so, the wheels arecompared, e.g., in pairs, diagonally, side by side, axle by axle, etc.This is performed in the circuit 6. The determined or estimatedtolerances G1(t,FS) are therefore a function of the actual travelsituation and of the progression (t) of the learning phase, taking intoaccount the predetermined correlations between the correction factorsK(t) or between the wheel speeds v(t).

The learning phase is concluded when the correction factors K(t,FS)reach a certain accuracy threshold G10, taking the actual travelsituation into account. It is also possible to define a plurality ofaccuracy thresholds G10, G10', G10", . . . , and to start the subsequentidentification phase as soon as a minimum accuracy threshold G10 hasbeen reached during the learning phase. The sensitivity threshold isthen raised when the next higher accuracy threshold G10' is reached, sothat even smaller differences in air pressure or smaller correctionfactor differences lead to the display of a loss of tire pressure.

The learning phase is followed by the identification phase (phase 2)shown in FIG. 3.

The travel situation recognition by means of a travel situationrecognition circuit 3' and the formation of the speed correction factorsK(t,FS) by means of a low-pass filter circuit 2' on the basis of thewheel speed signals v(t) and possibly of additional information are thesame as or similar to the corresponding steps in the learning phaseaccording to FIG. 2. The deviations G2(t,FS) of the current correctionfactors K(t,FS) from the values already learned, taking into account thetravel situation, are determined or estimated in a deviation circuit 7.The current deviations G2(t,FS) are compared in a comparison unit 11with sensitivity thresholds G20, G20', G20", and they are sent to anevaluation circuit 8. In the evaluation circuit 8, the current speedcorrection values K(t,FS) of the individual wheels are compared, takinginto account the current deviation G2(t,FS), with speed correctionfactors K_(Sp) already learned, i.e., determined during the learningphase for the instantaneous travel situation, taking into account theinstantaneous tolerance G1_(Sp), which has also been learned already.The learned values K_(Sp) and G_(Sp) are taken from memories 9 and 10.The evaluation circuit 8 recognizes a loss of tire pressure from thesignals sent to it when the condition

    K.sub.x (t,FS)+G2.sub.x (t,FS)<K.sub.Sp,x (FS)-G1.sub.Sp,x (FS)

is satisfied. The meanings of the individual terms are as follows:

x is wheel 1, wheel 2, wheel 3, wheel 4,

FS is the travel situation,

K_(x) (t,FS) is the current speed correction factor determined for wheelx during the identification phase (phase 2),

K_(Sp),x (FS) is the speed correction factor for wheel x, which isexpected in the instantaneous travel situation FS and was alreadydetermined in phase 1,

G1_(Sp),x (FS) is the tolerance already determined for the currenttravel situation FS during phase 1, and

G2_(x) (t,FS) is the current deviation for wheel x during phase 2.

The memories 9,10 are preferably reset each time a wheel is changed.This may be performed in the shop, or a hand switch can be provided forresetting. The sensitivity of the tire pressure monitoring now increasesas a function of the learning process, taking the corresponding travelsituation into account.

The deviations G2(t,FS) are compared with predetermined sensitivitythresholds G20, G20', G20", etc. The threshold G20 is in effect when aminimum accuracy threshold (G10) of the speed correction factors K(t,FS)is reached. If a higher accuracy had already been "learned," G20' is ineffect, while G20" is in effect at an even higher accuracy, etc.

We claim:
 1. Process for monitoring tire pressure by measuring,comparing and evaluating the rotating speeds of the individual wheels ofthe vehicle, wherein speed correction factors are formed for theindividual wheels of the vehicle in a learning phase according to alearning process, characterized in thatthe speed correction factors arevalues which, multiplied by the respective wheel speed, yield a basicspeed, correlations between the correction factors for typical travelsituations are determined, and tolerances as a function of therespective travel situation are determined for the correction factors;the deviations of the correction factors from the learned values aredetermined and evaluated in an identification phase, taking into accountthe travel situation-dependent tolerances and the progression of thelearning process, and loss of tire pressure is signaled when the currentcorrection factor for a wheel becomes smaller, taking into account thecurrent deviation, than the speed correction factor already determinedfor the wheel in question for the current travel situation during thelearning phase, taking into account the tolerance already determined forthe current travel situation during the learning phase.
 2. Process inaccordance with claim 1, characterized in that loss of tire pressure issignaled when the following condition is satisfied for a wheel:

    K.sub.x (t,FS)+G2.sub.x (t,FS)<K.sub.Sp,x (FS)-G1.sub.Sp,x (FS),

in which x is wheel 1, wheel 2, wheel 3, wheel 4, KS is the travelsituation, K_(x) (t,FS) is the current correction factor determined forwheel x during the identification phase, K_(Sp),x (FS) is the correctionfactor already determined during the learning phase for wheel x for thecurrent travel situation, G1_(Sp),x (FS) is the tolerance alreadydetermined in the learning phase for the current travel situation, andG2_(x) (t,FS) is the current deviation for wheel x during theidentification phase.
 3. Process in accordance with claim 2,characterized in that one or more of the following situations arerecognized as typical travel situations in which correlations betweenthe correction factors are determined and evaluated: Travel in curve,straight travel, high acceleration or deceleration of the vehicle, roughroad section, high drive slip or high wheel slip, high or lowcoefficient of friction, different coefficients of friction on the rightand left sides, etc.
 4. Process in accordance with claim 3,characterized in that the correlations are formed by comparison of thewheel speeds or of the correction factors in at least one of pairs, axleby axle, diagonally and side by side.
 5. Process in accordance withclaim 4, characterized in that the evaluation of the tolerances of thecorrection factors is performed according to accuracy thresholds reachedduring the learning phase as a function of the travel situation and ofthe progression of the learning process.
 6. Process in accordance withclaim 5, characterized in that the sensitivity thresholds are varied asa function of the travel situation.
 7. Process in accordance with claim5, characterized in that the sensitivity thresholds are varied as afunction of the travel situation and the progression of the learningphase.
 8. Process in accordance with claim 6, characterized in that thelearning phase is started by predetermined events.
 9. Process inaccordance with claim 8, characterized in that the learning phase isconcluded when the correction factors have reached, taking into accountthe actual travel situation or under ideal conditions, a predeterminedaccuracy threshold.
 10. Process in accordance with claim 9,characterized in that the identification phase is started only after theconclusion of the learning phase.
 11. Process in accordance with claim10, characterized in that the identification phase is started as soon asa minimum accuracy threshold is reached during the learning phase. 12.Process in accordance with claim 11, characterized in that a pluralityof accuracy thresholds are preset, and that the sensitivity threshold isvaried as a function of the accuracy threshold reached during thelearning phase.
 13. Circuit for carrying out the process in accordancewith claim 1, characterized in that it has a low-pass filter circuit forforming the speed correction factors on the basis of the wheel speedsignals; that the attenuation characteristic of the low-pass filtercircuit is variable as a function of the travel situation, which isreflected by the output signal of a travel situation recognitioncircuit; and that circuits for determining or estimating the travelsituation-dependent tolerances and circuits for determining orestimating the travel situation-dependent deviations from the speedcorrection factors are present.
 14. A circuit for monitoring the tirepressure of the wheels of a vehicle comprising:means for developingindications of the wheel speed of the individual wheels of the vehicle;a travel situation recognition circuit for developing an indication ofthe travel situation of the vehicle; a low-pass filter circuitresponsive to said wheel speed indications and said travel situationindication for forming speed correction factors, said low-pass filterhaving an attenuation characteristic which is variable as a function ofsaid travel situation indication; means responsive to said travelsituation indication for determining travel situation-dependenttolerances; and means for determining travel situation-dependentdeviations of said speed correction factors.